LOL! I'm honored to be asked this question! I know there is a firestorm of debate regarding this.

I think, like most great art, there are elements in the best cinema that are a form of a Rorshach test -- in that, we see what we want to see, hope to see.

We also have to take how the character was written and interpreted until that point into consideration.

Now, for my answer! LOL.

I do not believe it was Fredo. Yes, part of this is me not wanting to believe he could do it.

But a large part of it is me looking at what we have to go on--the evidence: how it was so brilliantly written by Puzo/Coppola and created/interpreted with such genius by Cazale.

Even at the height of his conflict with Michael -- there was no malice there, as interpreted by Cazale.

He was playing it from a deep place of hurt and wanting acceptance and wanting to prove himself. Never evil or fratricide.

We do know that there were 2 out of New York found dead there and Johnny Ola was in the house at some point.
I like to think it was one of those.

Hymen Roth was awfully resourceful, we have evidence of this. He could have had any of his associates pull that off. And we know from Fredo's "you guys lied to me" conversation with Ola that he was fooled. It is fair to assume when putting that conversation together with the "I didn't know it was gonna be a hit" conversation with Michael that, yes, Fredo really didn't know.

Fredo was guileless. Even when trying to be sneaky -- he can't pull it off. He wasn't even able to keep his "We've never met" lie going about Johnny Ola. What did that last--about a few hours?! LOL At most!

It's not natural to him. He doesn't have the heart for it. He also doesn't have the cunning for it. I'm reminded of Puzo's quote about Vito's 3 sons each getting a piece of him. Fredo got the heart. Michael got the reason. Sonny got the passion. I know someone here will have the exact quote - I am paraphrasing, obviously.

So, no, I don't think it could have been Fredo and that would leave one of Roth's men to do it. Which, I believe from what we've been shown by the writers and filmmakers, is not only entirely possible but very probable.

Now, let the agita over my answer begin!! hahaha
Cominciare!